Television and like system



lg- 1940- E. c. WHITE 2,

TELEVISION AND LIKE SYSTEM v Filed July 2, 1936 Del 04 Neil var)?INVENTOR ERIC L. C. WHITE BY 7% 5M ATTORNEY Patented Aug. 13, 1940UNITED STATES PAT NT OFFICE 2,210,995 TELEVISION AND LIKE- sY's'rEi'vlrEric Lawrence Gasling White, Middlesex, En

land, assignor to Electric & Musical Industries Limited, a. Britishcompany V Application July 2, 1936, Serial No. 88,528

In this specification, for convenience, the term- D. 0. component willbe applied to any or all of these low frequency components, and itshould be borne in mind that the D. C. component need not of necessityinclude a component of direct current.

It is known that when signals, such as television signals, which containa- D. C. component are amplified in an A. C. amplifier, or are passedthrough any other channel incapable of transmitting the D. C. component,this component is lost, and in. the output signal, variations of signalamplitude take place about what is known as the electrical zero; theelectrical zero is such that the areas enclosed by the graphrepresenting variations of signal amplitude with time, above and belowthe electrical zero respectively, are equal. Means which have beenproposed to compensate for this loss of the D. 0. component aredescribed in the specifications of British Patent No. 422,906 (U. S.Serial No. 720,205). and U. S. application Serial No. 40,532. r

In addition to causing loss of the D. 0. component, A. C. amplifiers andlike channels having a low-frequency cut-off may also cause a furtherform of distortion; the manner in which this distortion manifests itselfcan best be appreciated of the object from a consideration of the baseof a television signal, and for this purpose reference will now be madeto Fig. 1 of the accompanying drawing in which the signal voltage isplotted as ordinate against time as abscissa,

In Fig. 1, form of the television signal, the pulses a being linesynchronizing pulses and the parts b representing the picture signalscorresponding to lines being transmitted, and the horizontal dotted linedrepresents the average position of the datum, that is, the electricalzero line of the whole signal over a period of time substantially longerthan that shown.

A portion of the signal is shown after passing through an amplifier inwhich the 110. component is lost, and the figure shows the wave form inthe neighborhood of the point e where a relatively large change inaverage picture brightness from uniform dark grey to uniform white hastaken place. On the left of the point e, the signal the fullline-represents the wave.

Britain July .12, 11935 I (oi; risk-'6) is symmetrical about theelectrical zero, and on the right of point c it is tending to take up aposition 'in which it is again'symmetrical about the electrical zero.

Itswill be seen that on den change in average brightness, the signaldoes not at once take up a symmetrical position about the electricalzero; the signal rises to a relatively large amplitude, relative to theelectrical zero, and

thereafter the amplitude falls until the steady l symmetrical :state isreached; The dotted line e indicates approximately how the signalamplitude the occurrence of a sud decays overseveral line periodsimmediately after a sudden change in average brightness, will be notedthat the envelope of the signal through the peaks of the synchronizingpulses follows "a curve similar to'the curve e.

Now it will be observed that the picture signals on the left of point 0,in Fig. 1, and those on the extreme right of this figure,although'representing uniform grey and uniform white respectively, havea slight tilt from left to right towards the datum. I This tilt, whichis exaggerated. in the drawing for purposes of illustration, isintroduced during the passage of the-signals through theA; C. amplifierand is due to the fact that the amplifier has an imperfect response tolow-frequency signal components such as occur when the average picturebritghness changes and remains at or near. the new value, for a timewhich is comparable with the time constants of the couplings in theamplifier which determine the lowfrequency cut-off. Due to the decayindicated by dotted line e, the picture signals of each line, insteadfofsloping or left to right as do those tot-he left of point 0, are tiltedmuch more steeply, the amount of tilt decreasing towards the right untilfinally it is of the same order of magnitude as that of the signals tothe left of point c. The present invention is concerned with thedistortion of the type represented by the extra tilt introduced, in themanner discussed above, when there is asudden change in averagebrightness. The invention is, of. course, not limited to televisionsystems, and is applicablein other systems where distortion of the sametype arises.

Known methods of re-inserting the D. C. component leave the line signalswith the additional of the present inventionto provide a method oftilting only slightly from and it and means for removingor reducing thisdistor- The present invention accordingly provides a method of reducingor eliminating distortion of the type referred to, for use in televisionand like systems in which electrical signals are passed through achannel having a low-frequency cutoff, which method comprises the stepsof utilizing the distorted signals, during predetermined spaced periodsof time, to cause a reactance element to assume a charge dependent uponthe peak amplitude of said signals during said periods, 1

generating compensating electrical variations under the control ofchanges in the charge held by said reactance element, and mixing saidcompensating variations with said distorted signals in such sense and atsuch an amplitude as to reduce or eliminate said distortion.

Further features of the invention will appear from the followingdescription and appended claims.

An embodiment of the invention will now be described with reference toFig. 2 of the accompanying drawing, in which there is shown apparatusfor generating compensating variations which can be added to atelevision signal before the re-establishment of D. C. to compensate forthe additional tilt introduced, in the manner described above, when thesignal changes suddenly in average brightness. The television signal,which is without its D. C. component, and is of the nature of the signalshown in Fig, 1, is applied from terminal I between the control grid ofvalve 2 and earth and, through condenser 3, between the control grid ofvalve 4 and earth, it being arranged that the synchronizing signalsdrive the grids of valves 2 and 4 in the positive direction.

The control grid circuit of valve 4 contains a leak resistance l1, andthe flow of grid current in valve 4 when synchronizing pulses arepresent on its grid, causes condenser 3 to become charged to such anegative potential that no current flows in valve 4 until asynchronizing pulse arrives on its grid. By suitably determining thegrid bias of valve 4, the synchronizing pulses can thus be freed frompicture signals, the valve 4 being caused to pass current, that is, inother words, being switched on by each synchronizing pulse. The anode ofvalve 2 is connected to the positive terminal of a source (not shown) ofanode current, the negative terminal of which is earthed, the cathode ofvalve 2 is connected to the anode of valve 4 and the cathode of'thelatter valve is earthed. Means (not shown) are provided for biasing thecontrol grid of valve 2 to a negative potential relative to its cathode;valve 2 acts as a so-called cathode follower valve, the valve 4 servingas its cathode circuit impedance, and the arrangement being such thatthe potential of the cathode of valve 2 substantially follows variationsin the potential of its grid.

In operation, when a synchronizing pulse is present and valves 2 and 4are conducting, the cathode of valve 2 takes up a potential slightlynegative relative to the potential of the grid thereof; when there is achange in average brightness and the level of the tips of thesynchronizing pulses changes, a charge is given to condenser 5; thechange in the level of the tips of the pulses is, as has been explained,a measure of the amount of additional tilt for which it is desired tocompensate. The charge on condenser 5 maintains the cathode potential ofvalve 2 between pulses.

The potential set up across condenser 5 due to changes in the level ofthe tips of the synchronizing pulses, is applied to the grid of acathode follower valve 6, the cathode of which is connected throughresistance 7 to one pole of a condenser 8, the other pole of which isearthed.

Any change in the level of the tips of the synchronizing pulses, whichchange indicates the presence of additional tilt in the line signals,thus causes charging or discharging of the condenser 8; there are thusset up across the condenser 8 potential variations which can be employedto compensate for the addition tilt in the line signals.

The'signals from terminal I. are fed to valve I0, which is biased bymeans of resistance II to operate as an amplifier, and the potentialvariations set up across condenser 8 are reversed by valve 9 and fed toan amplifying valve l2 biased by resistance I3. Valves l0 and I2 have acommon anode resistance I 4, and these valves accordingly serve to mixthe television signals with the compensating anti-tilt variations. Themixed signals set up at terminals l5 are then fed to means, which may beof the kind described in the specification of co-pending applicationSerial No. 40,532, for re-inserting the D. C. component.

Since the anti-tilt variations may only commence to be generated inpractice after an interval of the order of one line period from thepoint at which the average brightness changes, the television signalsfed to valve I0 may be delayed by about one line period by means of adelay network I6.

The invention is not limited to the embodiment described and manymodifications within the scope of the appended claims will occur tothose versed in the art.

What is claimed is: V a

1. In television transmission wherein trains of video signals aredeveloped, each train representing a single line of a scanned object,and synchronizing signals are interposed between the trains of videosignals, an

trains of signals are passed through channels having a low frequencycut-off value and are distorted due to the abrupt large amplitudechanges of said video signals,'the method of reducing said distortionwhich comprises the steps of developing electrical energy of a valuedependent upon the peak amplitude of said signals, utilizing changes insaid developed electrical energy value to control the development ofcompensating signals, and mixing said compensating signals with saiddistorted signals in proper phase and amplitude so as to reduce saiddistortion,

2. In television transmission wherein trains of video signals aredeveloped, each train representing a single line of a scanned object,and synchronizing signals of a value outside the range of said videosignals are interposed between the trains of video signals, and whereinabrupt changes in amplitude of the video signals occur due to abruptchanges in the shade of the object along a linear portion thereof and inwhich said trains of signals are passed through channels having a lowfrequency cut-off value and are distorted due to the abrupt largeamplitude changes of said video signals, the method of reducing saiddistortion which comprises the steps of developing electrical energy ofa value dependent upon the peak amplitude of each successivesynchronizing pulse, maintaining said developed electrical energyconstant in value during the development of video signals, utilizingchanges in said developed electrical energy value to control thedevelopment of compensating signals, and mixing'said compensatingsignals with the distorted video Signals in proper phase and amplitudeso as to reduce said distortion. 3. Television transmission apparatuscomprising means for developing video signals having large amplitudechanges due to abrupt changes in shade of the scanned object along alinear scanned portion thereof, means for developing synchronizingsignals, amplifying means for said video and synchronizing signalshaving a low frequency cut-ofi value whereby said signals are distorteddue to the action of the amplifier means on the large abrupt amplitudechanges in the video signals, means for developing electrical energy ofa value dependent upon the peak amplitude of said video andsynchronizing signals, means for passing said signals from said lowfrequency channels to said electrical energy development means, meansfor generating compensating potentials, means for controlling saidcompensating potential generator in accordance with changes in thedeveloped electrical energy, and means for mixing said compensatingpotentials with said video and synchronizing signals whereby thedistortion due to the action of the low frequency cut-01f amplifieracting on the abruptly changing video signals is substantiallycompensated.

4. Television transmission apparatus comprising means for developingvideo signals, means for developing synchronizing signals having largeamplitude changes due to abrupt changes in shade of the scanned objectalong a linear scanned portion thereof, amplifying means for said videoand synchronizing signals having a low frequency cut-off value wherebysaid signals are distorted due to the action of the amplifier meanson'the large abrupt amplitude changes in the video signals, means fordeveloping electrical energy of a value dependent upon the peakamplitude of said video and synchronizing signals, means for passingsaid signals from said low frequency channels to said electrical energydevelopment means, means for generating compensating potentials, meansfor controlling said compensating potentialgenerator in accordance withchanges in the developed electrical energy, a time delay circuitinterposed between said distorted signal generator and said compensatingpotential generator, and means for mixing said compensating potentialswith said video and synchronizing signals whereby the distortion due tothe action of the low frequency cut-off amplifier acting on theabruptlychanging video signals is substantially compensated.

ERIC LAWRENCE CASLING WHITE.

